In vitro experiments, especially cell culture systems, have been extensively used for cytotoxicity studies. However, there are several major problems associated with conventional in vitro cytotoxicity assays that use static multi-well plate cultures. According to some such conventional methods, cells are cultured on two-dimensional surfaces, which may camouflage authentic cellular responses that would occur in vivo, where cells exhibit morphology and interact with each other in a three-dimensional milieu. According to other conventional methods, cells are cultured in suspension. A relatively long-term response to antigens or foreign materials (e.g., chemicals and manufactured nanomaterials) can rarely be measured due to metabolic-waste accumulation in the culture. Another disadvantage to both systems is that growth inhibition of the cultured cells is usually judged at an arbitrary point of time, lacking a dynamic long-term monitoring.
Cell culture methods that utilize 2-D cultures for cytotoxicity analyses are inherently prone to error because of the lack of a 3-D scaffold to support cell growth and proper tissue function. Reported research depicts the variation of cellular performance seen between 2-D and 3-D cell cultures including morphological, growth kinetics, growth factor expression, and other functional properties. It is evident that the 3-D culture conditions are important for in vivo-like differentiation, proliferation, metastatic potential of the tumor cells, and development of characteristic heterogeneity within the tumor population.
To mimic the native tissue for in vitro toxicity study models, the key concept is that there is a strong relationship between tissue structure and function. Therefore, in order to achieve the desired functional attributes in a tissue-engineered construct, the culture environment must represent the native counterpart. An important component in a tissue-engineered construct that allows for in vivo-like culture is a 3-D scaffold that allows cell population support, organization, and function.
So far, almost all cell based assays have been developed in 2-D culture systems, although conventional 2-D cultures usually suffer from contact inhibition and a loss of native cell morphology and functionality. In drug discovery, probably the least developed field is to reform in vivo tissue behaviors in in vitro assays, especially the development of in vitro models with cells in their unactivated or quiescent status. Currently, pharmaceutical firms spend large amounts of money on compound efficacy and cytotoxicity tests. However, there is still a 78% failure rate for all drugs, which may be devastating to developing companies. Effective compounds in vitro may be non-effective in vivo for many reasons, including differences between in vitro and in vivo target biology, interrelated biochemical mechanism, metabolism, poor penetration into solid tissues, etc. In comparison with 2-D cultures, 3-D cell models create a more realistic representation of real human tissues, which is critical to many important cell functions, including morphogenesis, cell metabolism, gene expression, differentiation and cell-cell interactions. Discrepancies in predicted drug treatment effectiveness in 2-D and 3-D cultures implicate the advantage of using 3-D culture systems. For cell-based sensing, particularly in studying cytotoxicity and drug discovery, maintaining cells in their native functional state in a proper 3-D environment would improve predictions and have the potential to reduce clinical trial failures.